Microtubule-based membrane transport is a complicated process that involves the mechanochemical enzymes dynein and kinesin as well as soluble activator proteins. Assays of organelle motility in vitro have allowed the identification and preliminary characterization of three distinct activators of dynein-driven organelle transport in chick embryo brain. The activators are likely to govern motor function in vivo. The goal of the proposed research is to elucidate the mechanism(s) by which two activators, 150/5OkD and C3, regulate motor activity and to initiate studies of regulation of motor activity in vivo. The following questions will be addressed. 1. How is cytoplasmic dynein activity regulated by the 150/5OkD activator? The 150/5OkD activator will be purified further and antibodies will be raised. The effects of the activator on dynein ATPase activity will be determined. Anti-activator antibodies will be used to immunolocalize the activator in cultured cells and in microinjection studies of live cells. 2. Do membranes affect enzyme activity? The membrane binding properties of dynein will be measured and it will be determined if membranes stimulate dynein ATPase. 3. Is cytoplasmic dynein activity regulated by phosphorylation? The C3 activator has protein kinase activity; it will be determined if cytoplasmic dynein is a substrate for phosphorylation. Dynein phosphorylation patterns will be analyzed to determine if they correlate with patterns of activity. 4. Is cytoplasmic dynein activity regulated during the cell cycle? Membrane traffic is known to be inhibited during mitosis; at this time, cytoplasmic dynein may be involved in the mitotic spindle movements. It will be determined whether "mitotic" dynein can serve as an organelle motor and how dynein activity is regulated during the cell cycle. These studies may provide insight into the role of dynein in mitosis.